Production of group iv, subgroup a, metal prills



Feb. 20, 1962 D. s. CHISHOLM ETAL PRODUCTION OF GROUP IV, SUBGROUP A,METAL PRILLS Filed April 1, 1957 HTTORNEYS 3,il21,562 PRODUCTEQN 6FGRUQUP EV, SUBGRGUP A, METAL PRILLS Douglas d. Qhishoim and Eon F. Hall,Midland, Mich, assignors to The Dow Qlternical Company, Midland, Nlich,a corporation oi Delaware Filed Apr. 1, 1957, Ser. No. saunas llilClaims. (ill. l847.2)

The invention is directed toward the production of a substantially pureductile metal of group IV, subgroup a, of the periodic table of elementsfrom an impure form thereof. It is particularly directed toward theproduction of such a metal in the form of prills by the electricarcmelting of impure ingots containing the metal.

Heretofore group IV, subgroup a, metals, hereinafter referred to assubgroup lVa, via, titanium, zirconium, hafnium, and thorium, have beenproduced largely as a sponge by the reduction of a halide of the metalto be produced in an atmosphere or aninert gas, e.g., helium or argon,by a metal more electropositive than the metal to be produced, e.g.,magnesium, sodium, or calcium. Thus produced, the sponge containsundesirable impurities. These impurities render the metal brittle andunsuitable Without purification for alloying and frabrication.

Substantially pure metals of subgroup IVa metals can be produced byreacting th impure metal with iodine to form the tetraiodide andthereafter thermally dissociating the tetraiodide by bringing it intocontact with a hot metal filament, e.g., tungsten or titanium, uponwhich the metal to be produced is deposited and from which the iodine isvolatilized. The filament is thereafter cooled and the subgroup lVametal thus deposited is scraped therefrom. The pr cess is slow andexpensive. Except for the preparation of a subgroup lVa metal for highlyspecialized uses, the tetraiodide dissociation process is noteconomical.

Therefore, the production of the subgroup lVa metals from their halidesby a reducing metal, such as magnesium, is currently in wide usealthough encumbered by a number of disadvantages. An improved method ofreducing a subgroup lVa metal from its halide by the action of magnesiumand recovering the metal, e.g., titanium or zirconiurn, is described inBritish Patent 734,166 and in our copending United States application,Ser. No. 315,664, filed October 20, 1952, now Patent No. 2,840,465. in asecond of our copending United States applications, Serial No. 653,391,filed April 17, 1957, now Patent No. 2,932,565, there is described animproved method of recovering the metal sponge produced according toBritish Patent 734,166 or copending application No. 315,604. Accordingto the second of the above identified copending applications, an ingotis produced comprising a core predominantly of the metal sponge beingproduced incased in a protective sheath predominantly of the halide ofthe reducing metal. The balance of the core is substantially the halideof the reducing metal, and the balance of the sheath is substantiallythe metal sponge being produced. At least 50 percent of the ingotconsists of the metal being produced. A core largely of titanium metalincased in a sheath largely of MgCl will illustrate an ingot thusformed.

Ductile metals of subgroup IVa, e.g., zirconium and titanium, in alloyedform are particularly suited to a wide number of uses requiring analloyed metal of high strength, medium weight, and excellent corrosionresistance. An example of such a use is in the manufacture of specificstructural aeroplane parts. Titanium or zirconium sponge made by methodscurrently in general use must be refined or purified before it cansatisfactorily meet the specifications required for alloys of suchmetals. An object of the invention is to provide a method of 3321,56?Patented Feb. 20, 1962 ice producing a substantially pure metal ofsubgroups IVa from an impure form thereof.

Another object is to produce a novel form of such substantially puremetal having the general shape of spheroids, herein referred to asprills.

A further object is to provide a method of producing such metals whichutilizes electric-arc melting of a feed stick or ingot comprising a coreof the metal to be produced protected both from air before entering thearc chamber and from the titanium tetrachloride vapors in the arcchamber by a sheath of a metal halide about the core.

A still further object is to produce a metal of subgroup lVa in the formof ductile prills which may subsequently be simultaneously compacted andresistance-sintered or welded into continuous rod for use as consumableelectrodes for alloying. v

A still further object is to provide a continuous method of producingductile metals of subgroup IVa which is more efficient and economicalthan methods now known.

These and other objects of. the invention will be made clear by thefollowing description of the invention together with the appended claimsand annexed drawing.

The invention is based upon the discovery that substantially puresubgroup IVa metal prills are produced from an impure form thereof whenan electric arc is struck in an atmosphere of an inert gas between twoelectrode rods or ingots which are predominantly of the metal to beproduced and one of which is rotating.

The invention is therefore directed to the novel method of producing aproduct consisting of substantially pure subgroup lVa prills. The methodbroadly consists of positioning two electrodes consisting of at least 50percent of the subgroup lVa metal to be converted into prills in an arcchamber, containing an inert gas, in such manner that a first electrodeis rotatable and a second electrode is advanceable toward the firstelectrode, rotating the first electrode and advancing the second towardit so as to strike an are between the two electrodes on applying asuitable current to them. Although the cross-section of the advanceableelectrode may be of any substantially symmetrical shape, it is preferredthat it be cylindriform. The rotatable electrode should be cylindriformand positioned so that it undergoes no substantial translatory motion.

The advanceable electrode must be of the impure subgroup IVa metal to beconverted into substantially pure metal. The metal of the rotatingelectrode may be alike or similar to that of the electrode advances-biletoward it or it may be substantially of the pure subgroup IVa metal tobe produced. It is preferred that it be substantially pure metal forreasons to be pointed out hereinafter. The subgroup IVa-bearingelectrodes are positioned in a non-parallel relationship so that theirlongitudinal axes form angles greater than 0 and less than 180 to eachother, but an angle of say 30 to gives better results than oneapproaching an angle of According to the preferred mode of practicingthe invention, the angle formed between the longitudinal axes should beabout 90, i.e., the advanceable electrode should be positionedsubstantially radially of the rotating electrode. It is also preferredthat the are be formed between the arcuate surface, near one end of therotating electrode, and the nose of the advanceable electrode. If thearc is struck too far back from either of the ends of the rotatingelectrode, the electrode becomes grooved and may melt through causingthe portion thereof which extends beyond the arc to drop oii. Therotating electrode may be elongated, but it is advantageous to have itsdisclike, i.e., its diameter large in comparison to its longitudinalaxis. It is also advantageous to have the rotating electrode of greatercircumference than the advanceabl electrode positioned generallyradially thereof so that greater peripheral speed may be attained at agiven angular speed. Successful operation permits a wide range of rateof rotation. An angular rate of rotation which results in a peripheralspeed of the rotatable electrode of between 10 and 100 feet per secondis usually employed.

Another mode of practicing the invention is to rotate both electrodes,either in opposite directions or in the same direction at differentspeeds. Still another mode is to oscillate the spatially stationaryelectrode by causing it to rotate, preferably through 360, in onedirection and then in the reverse direction an equal number of degrees.

FIGURE 1 shows an elevation partly in section of an apparatus suitablefor practicing the invention.

FIGURE 2 shows a cross-section of a rod or ingot of an impure subgroupIVa metal, e.g., titanium, made ac cording to copending applicationSerial No. 653,391, suitable as the consumable electrode for use in theinvention. It consists of a core lll, more or less rectangular incrosssection, consisting of roughly 80 to 90 percent metal spongeincased in cylindriform sheath 11 consisting of about 80 to 90 percentof the halide of the reducing metal.

FIGURE 3 shows sections of a consumable electrode of impure subgroup IVa metal placed end-to-end suitable for threaded engagement forcontinuous operation according to the invention.

FIGURE 4 shows a photomicrograph (magnification of 100 diameters) ofsectioned prills formed according to the invention.

Since an ingot consisting of a core predominantly of titanium and asheath predominantly of MgCl best typifies the material employed in thepractice of the invention, the invention will be illustrated by the useof such an ingot.

Referring to the drawing in more detail, FIGURE 1 shows an apparatussuitable for practicing a preferred mode of the invention. It comprisesadvanceable consumable electrode 1 of impure titanium, rotatingelectrode 2, and cylindrical arc-chamber 3 provided with conical bottom4, droptube 20, inert gas inlet 21, observation window 22, and suitabylocated openings for the operation of electrodes 2 and 3.

Electrode 1 passes through stuffing box 5 inserted in the openingprovided therefor in the wall of chamber 3 and made advanceable radiallyof electrode 2 near the foremost nose thereof to form gap 23 betweennose 24 of electrode 1 and arcuate surface 25 of electrode 2. Cop pernipple 6 contains electrical terminal 7 which is connected to lead-inline 8 leading to a source of D.C. or AC. Water jacket 9 about nipple 6is provided with water for cooling nipple 6. Electrode 1 is composed ofcore 10 and outer protective sheath 11.

Rotating electrode 2 is secured to shaft 12. Shaft 12 is positionedsubstantially horizontally and transversely in chamber 3 at right anglesto electrode 1. It is journaled in front and rear bearings 13, only therear one of which is shown supported just outside of chamber wall 14.Shaft 12 is rotated by pulley 15 turned by belt 16 which is powered by asource not shown. Shaft 12 is provided with central channel 17 throughwhich is circulated a cooling fluid. Shaft 12 also serves as anelectrical terminal, being connected by brush 18 to lead-in line 19which leads from the opposite pole of the source of electricityconnected to lead-in line 8.

Receiver 26 is shown removably affixed to drop tube 20 by means of bolts27. Valve 28 in droptube 20 and valve 29 in receiver 26 control the flowof reaction product from droptube 20 into receiver 26.

In practicing the invention according to the preferred mode ofoperation, inert gas is introduced through inlet 21 to flush out chamber3 and to maintain an atmosphere comprising the inert gas in the chamber.Lead-in lines 8 and 19 are connected to the opposite poles of a sourceof D.C. or AC. The voltage is not critical. A 60 volt D.C. and a voltA.C. have proved satisfactory. Consumable electrode 1 of an impuresubgroup IVa metal contaminated with a halide of a reducing metal isadvanced substantially radially of rotating electrode 2 which is of thesame subgroup IVa metal as advancing electrode 1, but which is in asubstantially pure state, so as to strike an arc in gap 23 formedbetween nose 24 of electrode 1 and arcuate surface 25 of electrode 2.

The proper positioning of electrode 1 to attain the desired arc may beaided by observing gap 23 through observation window 22. A suitable arcis usually obtained when the width of gap 23 is about /8 inch.

The heat provided by the arc melts and/or vaporizes electrode 1 at nose24 so that it is gradually and continuously consumed. Electrode 1 is,therefore, advanced either manually or automatically toward electrode 2.The metal is caused to fly from the are as small globules 3t) and thehalide salt either as vapor or droplets, both of which thereaftersolidfy. The moving surface of rotating electrode 2 dissipates the heatso that the melting point of rotating electrode 2 is not reached.Melting is therefore confined to non-rotating electrode 1. Furthermore,when the rotating electrode 2 is of substantially pure metal, itsconductivity is higher and therefore its temperature at the are issomewhat lower than that of the less pure metal.

It is preferable that electrode 2 rotate clockwise as shown in FIGURE 1to impel a large portion of the molten titanium and the halide saltupwardly so as to allow an added time to solidify before their strikingthe body of reaction product 34 at the bottom of chamber 3. It has beenobserved, however, that all particles of molten metal leaving the aresolidify into prills 31 within about 12 inches of the are. It is thoughtthat the halide first forms as a vapor, condenses into droplets 32 uponleaving the vicinity of the arc, and thereafter solidifies into granules33. The granules 33 and prills 31 fall into conical bottom 4 of chamber3 to form flowable reaction product 34 composed of a mixture of thegranules and prills. The dissipation of heat from the apparatus isusually sufiicient to cause halide droplets 32 to solidify beforebecoming part of reaction product 34, but if desired, cooling coils maybe placed about conical bottom 4 to aid in cooling.

By opening valves 28 and 29, reaction product 34 is dropped intoreceiver 26. Periodically valves 28 and 29 may be closed and bolts 27removed, thus disengaging receiver 26 without admission of air. Receiver26 is then either removed to a place of storage and replaced by anothersimilar receiver, or the contents properly disposed of and receiver 26again afiixed to droptube 20. Valves 28 and 29 are again opened.

Prills 31 are thereafter separated from the magnesium chloride by knownmethods as by dissolving the chloride in Water, collecting the prills ona suitable mesh sieve, e.g., a 100 mesh sieve (Standard Sieve of UnitedStates gureau of Standard Series), and washing with water and rymg.

The prills thus formed, from which the magnesium chloride has beenseparated, are substantially pure titanium. They have an average bulkdensity of about 3 grams per cubic centimeter. They are of a size thatat least 95 percent thereof will pass through a No. 6 sieve and beretained on a No. 200. They have a Brinell hardness number of betweenand 200.

They may be compressed into blocks or shapes. However, they are mostadvantageously employed in the preparation of consumable electrodes foralloying subgroup IVa metals by subjecting a mass of the prills topressure and simultaneously passing electric current through the mass toeffect a Welding together of the individual prills to form consumableelectrodes. A method by which particles of subgroup lVa metals may bemade into coherent massive metal suitable for use as a consumableelectrode is by the application of the principle described in ResistanceSintering Under Pressure in Journal of Metals, pages 158 to 167 (lanuary1955). If desired, the operation may be a continuous process. Aconvenient method by which the operation described therein may be madecontinuous to produce rods is described in Trans. of Am. Inst. of Miningand Metallurgical Engineers, vol. 171, Inst. of Metals Div., pages 416to 430 (1946), wherein a collet serves both as a means for providing theproper frictional resistance to the pressure imposed by a plungerelectrode on confined metal particles and as an electrode of oppositepolarity to the plunger.

Although the practice of the invention has been described for the use ofan ingot composed of a predominantly titanium core incased in apredominantly MgCl sheath, the invention is applicable to the productionof any subgroup IVa metal from an impure consumable electrode containingat least 50 percent of the metal to be produced.

Having described the invention, what is claimed and desired to beprotected by Letters Patent is:

l. The method of making ductile prills of a group IV, subgroup a, metalselected from the class consisting of titanium, zirconium, thorium andhafnium which comprises striking an electric arc in an atmosphere of aninert gas between the nose of an advanceable, consumable electrodeconsisting of at least 50 percent of the metal being produced and arotating cylindriform electrode of a metal having a melting point atleast as high as the metal being produced, said electrodes beingpositioned so that their longitudinal axes are substantially horizontaland form angles therebetween, each of said angles being less than 180.

2. The method of making substantially solid spheroidal prills of a groupIV, subgroup a, ductile metal selected from the class consisting oftitanium, zirconium, thorium and hafnium, which comprises striking anelectric are between the circumferential periphery of a rotatingcylindriform electrode consisting at least of 50 percent of the groupIV, subgroup a, metal being produced and the nose of an elongatedconsumable, substantially horizontal electrode advanceable toward therotating electrode, comprising the same said group IV, subgroup a, metalin an impure state, the longitudinal axis of said advanceable electrodebeing disposed to form angles, each of which is less than 180 with thelongitudinal axis of said rotating electrode, the nose of saidadvanceable electrode being positioned adjacent to but not touching theperiphery of said rotating electrode to maintain said electric arc.

3. The method according to claim 2 wherein the group IV, subgroup a,metal is titanium.

4. The method according to 2 wherein the metal of the advanceableelectrode contains magnesium chloride.

5. The method according to claim 2 wherein both electrodes comprise ametal sponge of group IV, subgroup a, and a halide of a metal moreelectropositive than titanium.

6. The'method according to claim 2 wherein said rotatable electrode issubstantially the pure metal of group 1V, subgroup a, being produced.

7. The method according to claim 2 wherein said advanceable electrodeconsists essentially of a predominantly titanium sponge core incased ina predominantly magnesium chloride sheath.

8. The method according to claim 2 wherein said advanceable electrodeconsists essentially of a predominantly zirconium sponge core incased ina predominantly magnesium chloride sheath.

9. The method according to claim 2 wherein the said advanceableelectrode is positioned radially of said rotating electrode.

10. The method according to claim 2 wherein the angles formed betweenthe longitudinal axes of the electrodes are each about References Citedin the file of this patent UNITED STATES PATENTS 513,270 Kreinsen Jan.23, 1894 1,133,508 Schoop Mar. 30, 1915 2,038,251 Vogt Apr. 21, 19362,189,387 Wissler Feb. 6, 1940 2,287,029 Dowdell June 23, 1942 2,518,720Richardson Aug. 15, 1950 2,619,776 Potters Dec. 2, 1952 2,678,879 Nueschet al. May 18, 1954 2,750,271 Cueilleron et al. June 12, 1956 2,753,255Alexander et al. July 3, 1956 2,787,534 Golwynne Apr. 2, 1957 2,795,819Lezberg et al. June 18, 1957 2,831,802 Raney Apr. 22, 1958 2,834,667Rostron May 13, 1958 2,897,539 McMillan Aug. 4, 1959 OTHER REFERENCESLanges Handbook of Chemistry, 9th edition, page 57.

2. THE METHOD OF MAKING SUBSTANTIALLY SOLID SPHEROIDAL PRILLS OF A GROUPIV, SUBGROUP A, DUCTILE METAL SELECTED FROM THE CLASS CONSISTING OFTITANIUM, ZIRCONIUM, THORIUM AND HAFNIUM, WHICH COMPRISES STRIKING ANELECTRIC ARC BETWEEN THE CIRCIMFERENTIAL PERIPHERY OF A ROTATINGCYCLINDRIFORM ELECTRODE CONSISTING AT LEAST OF 50 PERCENT OF THE GROUPIV, SUBGROUP A, METAL BEING PRODUCED AND THE